1. Field of the Invention
This invention relates to an improved xerographic reproduction process and an improved toner carrier used therewith.
2. Discussion of the Prior Art
It is known that by electrostatic means an image may be formed on the surface of a photoconductive material and developed. According to a basic xerographic method disclosed in the specification of U.S. Pat. No. 2,297,691, a uniform static charge is applied on a photoconductive insulating layer and the layer is exposed to light through a dark-light image, charge being discharged at light-exposed areas on the layer to form an electrostatic latent image. Development of the image is effected by attracting a fine, electrically charged material called "toner" to the latent image on the surface of the layer. Then, the powder image may be transferred to the surface of a support such as paper, where the transferred image is permanently fixed on the surface of the suppoert by, for instance, heating or the like. Instead of forming a latent image by uniformly charging the photoconductive layer and exposing the layer to light through a dark-light image, it is possible to form a latent image by charging the layer imagewise. It is also possible to omit the step of transferring the powder image whereby the powder image is fixed on the photoconductive layer. Of course, it is also possible to adopt a solvent treatment or coating treatment instead of the above heat fixing step.
Many methods are known for applying a toner to an electrostatic latent image. For instance, there is the "cascade" development method disclosed in the specification of U.S. Pat. No. 2,618,552. According to this method, the developing material may comprise a carrier powder of a relatively large size and the fine toner powder electrostatically attracted thereto. The developing material is rolled or cascaded across the latent image bearing surface. The material of the carrier powder is so selected that the toner powder is triboelectrically charged to a desired polarity. When the mixture of carrier powder and toner powder is rolled or cascaded across the image charged surface, the toner powder is electrostatically attracted to the charged area of the latent image, but not to the non-charged area of the image, namely the background area. The majority of the toner powder which happens to be applied to the background area is attracted back to the carrier and circulated again, because the electrostatic attraction between the toner and carrier is higher than the attraction between the toner and background area. This method is especially suitable for development of line images.
Another method for developing electrostatic images is the so-called "magnetic brush" method disclosed, for instance, in the specification of U.S. Pat. No. 2,874,063. According to this method, a developing material comprising a toner and a magnetic carrier powder is supported by a magnet, and the magnetic field of this magnet arranges the magnetic carrier in a brush-like form. This magnetic brush engages the electrostatic image bearing surface and the toner powder is drawn to the electrostatic image by electrostatic attraction.
The two foregoing methods have been generally adopted in the art. In the cascade method, small buckets on an endless belt raise developing material from a reservoir and carry it to the electrostatic image. The developer mixture is then cascaded or rolled across the electrostatic image bearing surface by gravity. All the carrier particles are then returned to the reservoir together with an unused toner powder, and circulated again through the development apparatus. In this method, the above procedure is repeated for each copy made by the machine, and in general, the above procedure is repeated several thousand times for the effective life of the developer. In this method and the magnetic brush method and other development methods, the developer mixture undergoes mechanical abrasion and is degraded by long-time use. Degradation of the carrier particles is characterized in that a part or all of the carrier coating is separated from the carrier core. The separated coating is either a fragment or the entire coating layer. The separation mainly results from a poor adhesive force between the coating material and its core. Separation mainly occurs upon collision or frictional contact with machine parts or other particles. A carrier having a coating which is likely to be separated from the carrier core in the form of a fragment, or otherwise must be replaced frequently thus undesirably increasing the copying costs both in terms of material expended and time consumed maintaining the machine. If carrier particles having a damaged coating are not replaced, printing defects result and printing a poor quality occurs.
In most carriers, the triboelectric and flow characteristics are adversely affected by high relative humidity. For instance, the triboelectric value of some carrier coatings varies with change in relative humidity, and such carriers are not suitable for use in a xerographic apparatus.
Another factor having influence on the stability of triboelectric characteristics of the carrier is that the carrier coating tends to undergo "toner impaction". That is, if the carrier particles are used in an automatic machine and are circulated several thousand times, many collisions occur between the carrier particles and other surfaces in the machine and tend to imbed toner powder. This may also occur because of other causes. As the amount of the toner permanently adhered to the carrier particle surface increases, the triboelectric value of the carrier particles changes and the amount of toner the carrier can carry is permanently lessened or destroyed, thereby deteriorating the quality of copies.
Further, the toner and carrier particles of a developing material should be selectively attracted to the electrostatic image and thus the toner must have the correct charge polarity and quantity. If the triboelectricity is too low, image background will be extremely contaminated. If triboelectricity is too high, the background will be clean and free from contamination, but the image concentration is so low that in some instances, the resulting image will be substantially illegible. In other words, there is an optimum range of triboelectricity for obtaining best overall results. A great variety of carriers are now used in automatic copying machines, but each of them has specific characteristics and most of them produce a high triboelectric charge on some kinds of toners and a low triboelectric charge on other kinds of toners. Thus, they are poor in overall characteristics.